With recent improvement and advancement of electronics technology for mobile PCs, mobile phones, personal digital assistants (PDAs) and the like, secondary batteries and the like, which can be repeatedly charged and discharged, are widely used as power storage devices for these electronic apparatuses. For these secondary batteries and other electrochemical power storage devices, electrode materials desirably have a higher capacity and a rapid charge/discharge property.
An electrode for such a power storage device contains an active material which is capable of ion insertion/desertion. The ion insertion/desertion of the active material is also referred to as doping/dedoping, and the doping/dedoping amount per unit molecular structure is referred to as dope ratio (or doping ratio). A material having a higher doping ratio can provide a higher capacity battery.
From an electrochemical viewpoint, the capacity of the battery can be increased by using an electrode material having a greater ion insertion/desertion amount. In lithium secondary batteries, which are attractive power storage devices, more specifically, a graphite-based negative electrode capable of lithium ion insertion/desertion is used in which about one lithium ion is inserted and deserted with respect to six carbon atoms to provide a higher capacity.
Of these lithium secondary batteries, a lithium secondary battery which has a higher energy density and is therefore widely used as the power storage device for the aforesaid electronic apparatuses includes a positive electrode prepared by using a lithium-containing transition metal oxide such as lithium manganese oxide or lithium cobalt oxide and a negative electrode prepared by using a carbon material capable of lithium ion insertion/desertion, the positive electrode and the negative electrode being disposed in opposed relation in an electrolyte solution.
However, this lithium secondary battery, which generates electric energy through an electrochemical reaction, disadvantageously has a lower power density because of its lower electrochemical reaction rate. Further, the lithium secondary battery has a higher internal resistance, so that rapid discharge and rapid charge of the secondary battery are difficult. In addition, the secondary battery generally has a shorter service life, i.e., a poorer cycle characteristic, because the electrodes and the electrolyte solution are degraded due to the electrochemical reaction associated with the charge and the discharge.
There is also known a lithium secondary battery in which an electrically conductive polymer, such as a polyaniline containing a dopant, is used as a positive electrode active material to cope with the aforesaid problem (see PTL1).
In general, however, the secondary battery employing the electrically conductive polymer as the positive electrode active material is of an anion migration type in which the electrically conductive polymer is doped with an anion in a charge period and dedoped with the anion in a discharge period. Where a carbon material or the like capable of lithium ion insertion/desertion is used as a negative electrode active material, it is therefore impossible to provide a rocking chair-type secondary battery of cation migration type in which a cation migrates between the electrodes in the charge/discharge. That is, the rocking chair-type secondary battery is advantageous in that a smaller amount of the electrolyte solution is required, but the secondary battery employing the electrically conductive polymer as the positive electrode active material cannot enjoy this advantage. Therefore, it is impossible to contribute to the size reduction of the power storage device.
To cope with this problem, a secondary battery of a cation migration type is proposed which is substantially free from change in the ion concentration of the electrolyte solution without the need for a greater amount of the electrolyte solution, and aims at improving the capacity density and the energy density per unit volume or per unit weight. This secondary battery includes a positive electrode prepared by using an electrically conductive polymer containing a polymer anion such as polyvinyl sulfonate as a dopant, and a negative electrode of metal lithium (see PTL2).